Modification of plastics with nano-scale particles

Adding solid fillers to a plastic is a classic way of modifying its properties. A large number of compounds have been developed in which, for example, chalk, carbon black, wood, rubber, glass and carbon fibres are incorporated into the plastic matrix. In most cases, the aim is to make the plastic harder, less brittle or more resistant to tensile, compressive or torsional stress.

If the internal interfaces between the plastic matrix and the filler are compatible, the properties of such compounds can be approximately described as a combination of the respective properties of the matrix plastic and the filler. If, however, the plastic is modified with nano-scale particles, the situation is very different. The reason is that, for a given quantity of solid particulate material, its surface area increases enormously as the size of the individual particles is reduced. Once the individual particles of the solid reach nano-scale dimensions, one gram of the material can easily have a surface area of several 100 m².

If such nano-scale particles can be mixed in significant quantities homogeneously into a plastic matrix, it can be assumed that the resultant material will have fundamentally different properties. It is then not the properties of the plastic matrix and the filler that, in combination, produce the properties of the resultant compound as is the case with traditionally filled plastic systems. Instead, the behaviour of these “nanocomposites” is determined by the special properties displayed by the polymer chains that form the plastic when these chains are located in the immediate vicinity of a nanoparticle surface. The surface of the particles considerably inhibits the polymer chains in terms of both their three-dimensional expansion and their shape. As a result they are no longer able – in the case of amorphous plastics – to freely assume their preferred entangled form or – in the case of semi-crystalline plastics – to freely form their crystallites.

Because, with nano-scale particles, an immense surface area is incorporated into the plastic, nearly all the polymer chains of the plastic – if they have been well dispersed – end up in the immediate vicinity of particles and are therefore significantly restricted in their ability to expand three-dimensionally. As a result, the plastic nanocomposite assumes a property profile corresponding to that of polymer chains hindered by interfaces. Depending on the matrix-filler combination, these properties can differ considerably from those obtained with a chemically identical compound containing a coarser-particle filler. This completely different behaviour of the plastic nanocomposite material, which is dominated by the effect of the internal interfaces, has inspired researchers and developers for many years to focus their work on these materials.

Potential objectives might include a further significant increase in hardness and scratch resistance in combination with improved strength and toughness (e.g. through the incorporation of nano-scale silica particles), reduced diffusion of gases and liquids through the plastic without it becoming brittle or non-transparent (e.g. through split lamella silicates such as mica), vastly increased ductility without loss of transparency (e.g. through nano-scale rubber phases), a higher refractive index for optical plastic parts (e.g. through titanium oxide or zirconium oxide nanoparticles), improved flow properties during processing, or higher tensile strength – possibly even anisotropic – as is expected, for example, of compounds with carbon nanotubes (CNTs).

The biggest challenge with all these plastic nanocomposites is to disperse the nano-scale filler in the plastic matrix so that the particles do not agglomerate to form aggregates. This often fails because the nanoparticle fillers emerge from their synthesis process as over-large structures of conjoined aggregates, or, during subsequent transport and storage, "cake together" to form such aggregates. In the majority of cases, it is then no longer possible to break up these agglomerated particles during the compounding process, with the result that the properties of the final material are well below their potential. Recent developments nevertheless offer hope that more efficient ways will soon be found to achieve effective re-dispersion.

Even if this does prove successful, the problem remains that a plastic nanocomposite in which nano-scale particles are dispersed is a long way away from its preferred state of thermodynamic equilibrium. The polymer chains, which nearly all suffer from the influence of the large internal interfaces in the nanocomposite, want to free themselves from these restrictions as effectively as possible and regain their unhindered chain form. They succeed in doing this during flow processes and thus above all with the support of the shear forces occurring during forming (extrusion, injection moulding). This results in phase separation of the plastic and the nanoparticles, leading to agglomeration of the previously dispersed nanoparticles to form aggregates. To prevent this, further innovation is needed.

An additional challenge presented by plastic nanocomposites apart from effective particle distribution and stabilisation in the dispersed form is the production and handling of the nanoparticle fillers themselves. Measures must be taken to ensure that they do not escape into the environment and into the air where they may be inhaled. This could require new concepts. Once the nanoparticles have been integrated into the plastic matrix, however, it can be assumed that the potential health and environment risks associated with the nano-size of the particles are minimal.

In all, plastics containing nano-scale fillers currently represent one of the biggest challenges for R&D in the plastics sector. As is documented not only by theoretical studies but also by some impressive practical examples, they have immense potential for the development of highly innovative materials, in some cases with completely new sets of properties. It would certainly seem worthwhile performing further intensive research on this subject. On the other hand, the responsible handling of these materials will also require serious assessment of the potential risks. After weighing up the various aspects, it should become clearer what new fields of application will open up for these fascinating new modified plastics.

Modification of plastics with nano-scale particles - Vita Prof. Dr. Matthias Rehahn